#include <iostream>
#include <vector>
using namespace std;

class Solution {
public:
    int missingNumber(vector<int>& nums)
    {
        int ret = 0;
        for (auto& e : nums)
            ret ^= e;
        for (int i = 0; i <= nums.size(); ++i)
            ret ^= i;

        return ret;
    }
};

int main()
{
    Solution();
	return 0;
}